CVE-2024-35917 s390/bpf: Fix bpf_plt pointer arithmetic

2024-05-1908:35:09

Linux

github.com

cve-2024-35917

s390/bpf

fix bpf_plt pointer

kernel vulnerability

pointer arithmetic

kui-feng lee

crash report

gcc reordering

null pointers

alias analysis

c standard violation

assembly offsets fix

AI Score

Confidence

Low

EPSS

Percentile

15.5%

SSVC

Exploitation

none

Automatable

Technical Impact

partial

JSON

In the Linux kernel, the following vulnerability has been resolved:

s390/bpf: Fix bpf_plt pointer arithmetic

Kui-Feng Lee reported a crash on s390x triggered by the
dummy_st_ops/dummy_init_ptr_arg test [1]:

[<0000000000000002>] 0x2
[<00000000009d5cde>] bpf_struct_ops_test_run+0x156/0x250
[<000000000033145a>] __sys_bpf+0xa1a/0xd00
[<00000000003319dc>] __s390x_sys_bpf+0x44/0x50
[<0000000000c4382c>] __do_syscall+0x244/0x300
[<0000000000c59a40>] system_call+0x70/0x98

This is caused by GCC moving memcpy() after assignments in
bpf_jit_plt(), resulting in NULL pointers being written instead of
the return and the target addresses.

Looking at the GCC internals, the reordering is allowed because the
alias analysis thinks that the memcpy() destination and the assignments’
left-hand-sides are based on different objects: new_plt and
bpf_plt_ret/bpf_plt_target respectively, and therefore they cannot
alias.

This is in turn due to a violation of the C standard:

When two pointers are subtracted, both shall point to elements of the
same array object, or one past the last element of the array object
…

From the C’s perspective, bpf_plt_ret and bpf_plt are distinct objects
and cannot be subtracted. In the practical terms, doing so confuses the
GCC’s alias analysis.

The code was written this way in order to let the C side know a few
offsets defined in the assembly. While nice, this is by no means
necessary. Fix the noncompliance by hardcoding these offsets.

[1] https://lore.kernel.org/bpf/[email protected]/

CNA Affected

[
  {
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "vendor": "Linux",
    "product": "Linux",
    "versions": [
      {
        "status": "affected",
        "version": "f1d5df84cd8c",
        "lessThan": "c3062bdb859b",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "f1d5df84cd8c",
        "lessThan": "d3d74e45a060",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "f1d5df84cd8c",
        "lessThan": "7ded842b356d",
        "versionType": "git"
      }
    ],
    "programFiles": [
      "arch/s390/net/bpf_jit_comp.c"
    ],
    "defaultStatus": "unaffected"
  },
  {
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "vendor": "Linux",
    "product": "Linux",
    "versions": [
      {
        "status": "affected",
        "version": "6.3"
      },
      {
        "status": "unaffected",
        "version": "0",
        "lessThan": "6.3",
        "versionType": "custom"
      },
      {
        "status": "unaffected",
        "version": "6.6.26",
        "versionType": "custom",
        "lessThanOrEqual": "6.6.*"
      },
      {
        "status": "unaffected",
        "version": "6.8.5",
        "versionType": "custom",
        "lessThanOrEqual": "6.8.*"
      },
      {
        "status": "unaffected",
        "version": "6.9",
        "versionType": "original_commit_for_fix",
        "lessThanOrEqual": "*"
      }
    ],
    "programFiles": [
      "arch/s390/net/bpf_jit_comp.c"
    ],
    "defaultStatus": "affected"
  }
]